Fiber optic networks are becoming increasingly popular for data transmission due to their high speed, high capacity capabilities. Multiple wavelengths may be transmitted along the same optic fiber. These wavelengths are sent combined to provide a single transmitted signal. A crucial feature of a fiber optic network is the separation of the optical signal into its component wavelengths, or "channels", typically by a wavelength division multiplexer. This separation must occur in order for the exchange of wavelengths between signals on "loops" within networks to occur. The exchange occurs at connector points, or points where two or more loops intersect for the purpose of exchanging wavelengths.
Add/drop systems exist at the connector points for the management of the channel exchanges. The exchanging of data signals involves the exchanging of matching wavelengths from two different loops within an optical network. In other words, each signal drops a channel to the other loop while simultaneously adding the matching channel from the other loop.
FIG. 1 illustrates a simplified optical network 100. A fiber optic network 100 could comprise a main loop 150 which connects primary locations, such as San Francisco and New York. In-between the primary locations is a local loop 110 which connect with loop 150 at connector point 140. Thus, if local loop 110 is Sacramento, wavelengths at San Francisco are multiplexed into an optical signal which will travel from San Francisco, add and drop channels with Sacramento's signal at connector point 140, and the new signal will travel forward to New York where it is demultiplexed into individual wavelengths. Within loop 110, optical signals would be transmitted to various locations within its loop, servicing the Sacramento area. Local receivers (not shown) would reside at various points within the local loop 110 to convert the optical signals into the electrical signals in the appropriate protocol format.
The separation of an optical signal into its component channels are typically performed by a dense wavelength division multiplexer. FIG. 2 illustrates add/drop systems 200 and 210 with dense wavelength division multiplexers 220 and 230. An optical signal from Loop 110 (.lambda..sub.1 -.lambda..sub.n) enters its add/drop system 200 at node A (240). The signal is separated into its component channels by the dense wavelength division multiplexer 220. Each channel is then outputted to its own path 250-1 through 250-n. For example, .lambda..sub.1 would travel along path 250-1, .lambda..sub.2 would travel along path 250-2, etc. In the same manner, the signal from Loop 150 (.lambda..sub.1 '-.lambda..sub.n ') enters its add/drop system 210 via node C (270). The signal is separated into its component channels by the wavelength division multiplexer 230. Each channel is then outputted via its own path 280-1 through 280-n. For example, .lambda..sub.1 ' would travel along path 280-1, .lambda..sub.2 ' would travel along path 280-2, etc.
In the performance of an add/drop function, for example, .lambda..sub.1 is transferred to path 280-1. It is combined with the others of Loop 150's channels into a single new optical signal by the wavelength division multiplexer 230. The new signal is then returned to Loop 150 via node D (290). At the same time, .lambda..sub.1 ' is transferred to path 250-1 from 280-1. It is combined with the others of Loop 110's channels into a single optical signal by the wavelength division multiplexer 220. This new signal is then returned to Loop 110 via node B (260). In this manner, from Loop 110's point of view, channel .lambda..sub.1 of its own signal is dropped to Loop 150 while channel .lambda..sub.1 ' of the signal from Loop 150 is added to form part of its new signal. The opposite is true from Loop 150's point of view. This is the add/drop function.
Conventional methods used by dense wavelength division multiplexers in separating an optical signal into its component channels includes the use of filters and fiber gratings as separators. A "separator," as the term is used in this specification, is a unit of optical components which separates one or more channels from an optical signal. Filters allow a target channel to pass through while redirecting all other channels. Fiber gratings target a channel to be reflected while all other channels pass through. Both filters and fiber gratings are well known in the art and will not be discussed in further detail here.
A problem with the conventional separators is the precision required of a device for transmitting a signal into an optic fiber. A signal entering a dense wavelength division multiplexer has a very narrow pass band. FIG. 3 shows a sample spectrum curve 310 of channels as it enters a dense wavelength division multiplexer. The pass band 320 of the channels are very narrow. Ideally, the curve would be a square wave. A narrow pass band is problematic because due to the physical limitations and temperature sensitivity of signal transmitting laser devices, they never transmit light exactly to the center wavelength of an optic filter. The amount off center is called the "offset." The amount of drift ideally should not be larger than the width of the pass band. Otherwise, crosstalk between channels will be too large. Crosstalk occurs when one channel or part of a channel appears as noise on another channel adjacent to it. Since the signals resulting from the conventional configurations have a narrow pass band, the signal transmitting devices, such as lasers or the like, must be of a high precision so that offset is limited to the width of the pass band. This high precision is difficult to accomplish. Signal transmitting devices of high precision is available but are very expensive. Also, the signal transmitting devices much be aligned individually for each separator, which is time intensive.
Therefore, there exists a need for a wavelength division multiplexer with a method of separation which has a greater tolerance for wavelength offset and is easier to align. This method should also be cost effective to implement. The present invention addresses such a need.